The present invention relates to Ziegler-Natta catalyst compositions for use in the polymerization of olefins having improved control over polymer properties through the use of carefully chosen mixtures of selectivity control agents. Ziegler-Natta catalyst compositions are well known in the art. Typically, these compositions include a transition metal polymerization catalyst, especially titanium containing compound; a co-catalyst, usually an organoaluminum compound; and a selectivity control agent (SCA), usually an organosilicon compound. Examples of such Ziegler-Natta catalyst compositions are shown in U.S. Pat. No. 4,107,413; U.S. Pat. No. 4,294,721; U.S. Pat. No. 4,439,540; U.S. Pat. No. 4,115,319; U.S. Pat. No. 4,220,554; U.S. Pat. No. 4,460,701; U.S. Pat. No. 5,247,032; U.S. Pat. No. 5,247,031; U.S. Pat. No. 5,229,342; U.S. Pat. No. 5,153,158; U.S. Pat. No. 5,151,399; U.S. Pat. No. 5,146,028; U.S. Pat. No. 5,106,806; U.S. Pat. No. 5,082,907; U.S. Pat. No. 5,077,357; U.S. Pat. No. 5,066,738; U.S. Pat. No. 5,066,737; U.S. Pat. No. 5,034,361; U.S. Pat. No. 5,028,671; U.S. Pat. No. 4,990,479; U.S. Pat. No. 4,927,797; U.S. Pat. No. 4,829,037; U.S. Pat. No. 4,816,433; U.S. Pat. No. 4,728,705; U.S. Pat. No. 4,562,173; U.S. Pat. No. 4,548,915; U.S. Pat. No. 4,547,476; U.S. Pat. No. 4,540,679; U.S. Pat. No. 4,472,521; U.S. Pat. No. 4,442,276; and U.S. Pat. No. 4,330,649.
Catalyst compositions designed primarily for the polymerization of propylene or mixtures of propylene and ethylene generally include a selectivity control agent in order to affect polymer properties, especially tacticity or stereoregularity of the polymer backbone. As one indication of the level of tacticity, especially the isotacticity of polypropylene, the quantity of such polymer that is soluble in xylene or similar liquid that is a non-solvent for the tactic polymer is often used. This is referred to as the xylene solubles content of the polymer, or XS. In addition to tacticity control, molecular weight distribution (MWD), melt flow (MF), and other properties of the resulting polymer are affected by use of a SCA as well. Because MF is also affected by use of a chain transfer agent, normally hydrogen, the H2 response of the polymerization can be adjusted through the use of a SCA. Often however, a SCA which gives desirable control over one polymer property, is ineffective or detrimental with respect to additional properties of the polymer.
Use of mixtures of SCA's in order to adjust polymer properties either according to an expected average of the resulting properties or through use of multiple reactors, thereby achieving benefit of the effect of individual SCA's is known. Examples of prior art disclosures of catalyst compositions making use of mixed SCA's include: U.S. Pat. No. 5,652,303, U.S. Pat. No. 6,087,459, U.S. Pat. No. 6,147,024, U.S. Pat. No. 6,111,039, WO 95/21203 and WO 99/20663. Disadvantageously, certain highly desirable SCA's, referred to as “dominating SCA's”, generally operate under polymerization conditions so as to exclude the effects of other SCA's. If a normally dominating SCA is present in a single reactor under conventional polymerization conditions, the resulting polymer properties are determined essentially solely by the dominating SCA, and little or no effect from the presence of the additional SCA's is observed. Other SCA's, referred to as “competing” or “cooperative” SCA's, operate in mixtures according to an expected mutuality wherein both compounds affect the polymer properties. Just as the relative rate constants of different compounds determines their relative productivities under use conditions, various polymer properties are affected to a greater or lesser degree by different SCA's. With respect to a given polymer property or functionality, the corresponding ability of a given SCA to affect that property operating as the sole SCA or as a mixture of one or more SCA's can be measured. Based on such measurements, the relative functionality control capability of individual compounds and the relative dominating ability of SCA's when employed as a mixture can be determined.
Previously, the use of mixtures of SCA's in a single reaction step and reactor has been confined to the use of cooperative mixtures for the foregoing reason. Examples of such cooperative mixtures of SCA's include the combination of DCPDMS and propyltriethoxysilane (PTES) or methylcyclohexyldimethoxysilane (MCHDMS). Other examples are disclosed in U.S. Pat. No. 6,337,377, U.S. Pat. No. 6,303,698, U.S. Pat. No. 6,184,328, U.S. Pat. No. 6,133,385, U.S. Pat. No. 6,127,303, U.S. Pat. No. 6,096,844, U.S. Pat. No. 6,087,459, U.S. Pat. No. 6,066,702, U.S. Pat. No. 5,869,418, U.S. Pat. No. 5,849,654, U.S. Pat. No. 5,844,046, U.S. Pat. No. 5,652,303, U.S. Pat. No. 5,414,063, U.S. Pat. No. 5,192,732, U.S. Pat. No. 5,100,981, and WO 99/58585.
WO 95/21203 recognized dominating behavior for SCA's, when used together in a single reaction step, at molar ratios of SCA: transition metal of 33:1. Mixtures of dicyclopentadienyldimethoxysilane (DCPDMS) and tetraethoxysilane (TEOS) were used as the SCA pair of interest. U.S. Pat. No. 6,111,039 and WO 99/20663 disclosed the use of a multistage process for preparing α-olefin homopolymers and copolymers, especially polypropylene and ethylene/propylene copolymers, using mixtures of SCA's wherein one SCA is dominating. The dominating effect of one SCA over the other was avoided by adding the dominating SCA to only the second of a series of reactors.
There remains a need in the art to provide a Ziegler-Natta catalyst composition for the polymerization of olefins comprising the combination of a Ziegler-Natta catalyst with a mixture of selectivity control agents including in said mixture, a dominating selectivity control agent, characterized in that the activity of a dominating selectivity control agent is moderated, and the properties of the resulting polymer are influenced by all of the SCA's in the mixture.